The present invention relates to an optical device; and, more particularly, to a periodically poled optical fiber and a method for the manufacture thereof.
Optical fibers go through the poling process for obtaining second-order nonlinear optical properties. In the general process of poling, a metal electrode(s) is formed at one or both faces of a D-shaped optical fiber, given voltage at a high temperature for a predetermined period, and then cooled down slowly with the voltage still maintained.
Although optical fibers have a big second-order non-linear coefficient, the phase matching condition between interacting waves should be satisfied. As optical fibers also have the effect of dispersion, which makes the refractive index different according to each wavelength, the phase matching condition usually fails to be achieved between lights with different frequencies. The phase matching can be achieved, though, by adding an absorber, which absorbs light at a certain wavelength and thereby making the refractive index get reversed with respect to the wavelength at around the resonance absorption wavelength, or by using the birefringence of optical fibers. In this way, however, it""s hard to fulfill the condition, and the wave range fulfilling the phase matching is extremely limited. Therefore, in order to use the second-order nonlinearity at a required wavelength the phase matching condition needs to be actively controlled, which eventually leads to the introduction of the quasi phase matching condition.
Generally, in second-order nonlinear single crystal, the poling direction is turned at an angle of 180 degree with respect to adjacent domain periodically. So, optical fibers also need to be poled periodically, preferably in opposite poling direction to the adjacent domains.
In FIGS. 1A to 1C, there are shown cross-sectional views setting forth a conventional method for fabricating poled optical fibers, described in U.S. Pat. No. 5,617,499, entitled to xe2x80x9cTECHNIQUE FOR FABRICATION OF A POLED ELECTROOPTIC FIBER SEGMENT,xe2x80x9d which is incorporated herein by reference.
As shown in FIG. 1A, a first electrode 12 is formed on top of a silicon substrate 11, then an adhesive layer 13, made of polyimide, is formed on top of the first electrode 12 and the silicon substrate 11 for adhering a D-shaped optical fiber 10 to the first electrode 11. And then, as shown in FIG. 1B, a first insulation layer 14, made of polyimide, is formed on top of the entire structure to both fix the fiber position and provide a dielectric material so that high electric fields can later be applied across the D-shaped optical fiber.
As shown in FIG. 1C, the first insulation layer 14 and the D-shaped optical fiber 10 are polished to provide a planar surface, then a second electrode 15 is formed on the optical fiber, and then a second insulation layer 16 and metal layer 17 are deposited thereon.
As shown above, in the conventional method of fabricating periodically poled optical fibers, an optical fiber should go through the complex procedures of being processed into one with its cross-section in D-shape, fixed on the substrate, deposited over with a photoresist and removed of, then masked, and then deposited with an electrode material. So, the conventional method has a shortcoming of the complicacy of depositing the microelectrode pattern on a section of an optical fiber. Moreover, an electrode cannot be used again once used in forming polarization of an optical fiber.
Meanwhile, as shown in FIG. 2, there are another method of inserting electrodes into two holes 22 around the core C of the optical fiber 20 from the opposite direction to each other, giving it voltage at a steady high temperature, sometimes irradiating a laser of ultraviolet wavelength, then cooling it down slowly after maintaining it for a predetermined period, thereby fixing the poling condition induced to the core of the optical fiber. These methods have a shortcoming that a domain poled periodically cannot be formed while it is possible to pole in the same direction over the entire optical fiber where a voltage is given to through electrodes. That is, the conventional fabrication method of periodically poled D-shaped optical fiber makes adjacent domains remain unpoled. Moreover, it is very hard to induce the already poled domains and the adjacent domains poled in the opposite direction to each other.
It is, therefore, an object of the present invention to provide an electrode for fabricating periodically poled optical fibers, which make the fabrication process relatively simple and inexpensive.
In accordance with an embodiment of the present invention, there is provided a method for fabricating poled optical fibers, which can form consecutive and further periodically inverting poling domain, thus increasing the second-order nonlinerity of optical fibers.
In accordance with an aspect of the present invention, there is provided an electrode for fabricating periodically poled optical fibers, which comprises a plurality of grooves on one or more surfaces.
In accordance with an aspect of the present invention, there is provided an electrode for fabricating periodically poled optical fibers, provided with a plurality of opening holes formed at a regular period.
In accordance with an aspect of the present invention, there is provided a method for fabricating periodically poled optical fibers using a first electrode made in accordance with anyone of claims 1 to 4, the method comprising the steps of: a) forming an optical fiber, which includes a lower clad layer, a core and an upper clad layer, provided with electrode arrangement space in the respective lower and the upper clad layers; b) placing the first electrode in one of the lower and upper clad layers and then placing a second electrode in the other clad layer; c) forming a plurality of a first poling domain by giving voltage to each the first and the second electrodes; d) re-arranging the first electrode and the second electrode so that domain not poled during the step c) could be poled; and e) forming a second poling space between each of the first poling spaces by giving voltage to the first and the second electrodes.